Add REST API endpoint for RepositoryIndexServlet

Added support for the JSON output format to RepositoryIndexServlet and
RefServlet.

Sample JSON response from RepositoryIndexServlet:
  )]}'
  {
      "name": "gitiles",
      "clone_url": "https://gerrit.googlesource.com/gitiles",
      "description": "A simple browser for Git repositories"
  }

Sample JSON response from RefServlet:
  )]}'
  {
    "HEAD": {
        "value": "99df26787bf9d8b78bef8c12ad821d7a8fdfa044",
        "target": "refs/heads/master"
    },
    "refs/heads/32.93.1611": {
        "value": "e50434a901d89cd40ff1809b40a97bca87de2b3f"
    },
    "refs/tags/cut/2.8.2": {
        "value": "25ec7478b5f2d39d78a8978e727cd089e9af9df3",
        "peeled": "7ca778c7451560b27b3c902b21c528b0dde772d6"
    }
  }

Change-Id: I83caa5a46314b5d16bb6c14d275d191f1e888cc1
2 files changed